This invention relates to a hip joint prosthesis. More particularly, this invention relates to an endoprosthesis having a compliant head that conforms to the shape of the acetabulum of the patient.
Replacement of diseased or otherwise impaired natural joints of the human skeleton or their components by artificial prosthetic devices has gained an extremely wide acceptance in the field of orthopaedics. Endoprostheses generally have a stiff spherical or slightly out-of-round head. All attempts to produce a soft endoprosthesis using plastic materials have been unsuccessful.
The hip joint is a ball and socket joint of the type required to allow rotation about all three axes. While it is clear that two spherical surfaces satisfy the requirement of three axes of rotation, and both the femoral head and the acetabulum of the hip joint appear to be quite spherical when unloaded, it is not clear that these unloaded shapes are maintained under load, particularly so because of the variations in thickness of the two cartilage layers.
Presently available measurement techniques have determined that the cartilage surface of the acetabulum is very spherical with maximum deviations from sphericity on the order of about 150 .mu.m and that the surface of the underlying bone of the acetabulum is spherical to within about 500 .mu.m. In addition, these measurements show that the cartilage surface of the femoral head is very spherical with maximum deviations of about 200 .mu.m occurring on very localized areas adjacent to the cartilage edges and over most of the surface being less than about 100 .mu.m while the maximum deviations from sphericity of the femoral bone surface are on the order of about 500 .mu.m. As is well known in anatomy, the cartilage thickness distribution on the femoral head and on the acetabulum are reciprocal in the sense that a thicker cartilage on the acetabulum corresponds (in erect posture) to thinner cartilage on the femoral head. As the first approximation, local compliancy of the cartilage layer can be considered proportional to its local thickness. Most of the thickness variation can be accounted for by the relative shift of the best-fitting spheres to the cartilage surface and to the cartilage-to-bone interface. Consistency of this shift suggests physiological significance of the cartilage thickness distribution.
Accordingly, it would be desirable to provide an endoprosthesis that approaches the compliancy of the natural femoral head as nearly as possible both in the magnitude and distribution. In addition, it would be desirable to provide such an endoprosthesis which is, by adjusting its shape under the normal load to which the joint is subjected, capable of compensating for unavoidable errors in choosing the proper size of the replacement.